The increasing proportion of automation for twenty five years in aviation, both civil and military, is leading aircraft crews more and more to carry out tasks of anticipation and of monitoring of the execution of the mission and of the electronic flight management systems, and less and less to directly influence the primary aircraft flight controls.
This trend has been accentuated these last twenty years with the spread of onboard flight management systems (commonly called FMS).
These systems hold a large number of data:
                originating from sensors (GPS, VHF) for navigation,        originating from databases (navigation databases) for generating the electronic flight plan,        originating from performance databases for generating the predictions along the flight plan,        originating from route instruction, constraint or strategy entries made manually by the crew (usually in order to initialize the computations) or automatically by digital data link coming from the airline or from control centres (ATC).        
Amongst the frequent and repetitive tasks carried out by the crew so as to always benefit from the air traffic assistance and control services, there is the selection of the audio frequency on the VHF radio equipment which has to be done on each change of control sector. The transfer between sectors is carried out following the receipt of a flight instruction, hereafter called “clearance”, received from the controller of the current sector for making contact with the next sector, at the boundary of two sectors. As long as the frequency reassignment has not been made by the current controller, the aircraft is his responsibility, even if he is geographically in another sector.
Because of the increasing number of aircraft per sector and of the physical limit to the aircraft that can be handled by one controller, the sectors are increasingly smaller, which induces a larger number of sectors and hence of frequencies to be contacted. This is made possible thanks to a recent increase in frequencies linked to the reduction from 25 kHz to 8.33 kHz of the spaces between the latter, despite the restricted size of the bandwidth available for audio aviation communications. Therefore, the work of frequency transfer is increasingly time-consuming both for the controller and for the pilot. Furthermore, these more frequent verbally-made frequency transfers cause an increase in communications and hence mechanically a congestion on the frequency that may prejudice the control instructions and safety communications.
This may cause very dangerous situations for the controller who “sees” an aircraft on his radar screen in his sector, but has no control of it because he does not have its frequency (and can therefore not communicate directly and rapidly with it). Currently, this problem is solved by organizing ATC control rooms so that the air traffic controllers of adjacent sectors are close together and can verbally remind a colleague that he has forgotten to transfer him an aircraft, or, even if he is in another centre, communicate with him via a telephone call.
Studies are currently being carried out to smooth the load of the controller, with the objective of optimizing the partitioning of the sectors, their combination and their division in a dynamic manner. Specifically, there exists, particularly in Europe, a real problem of frequency congestion and traffic density. The “transfer” messages form the majority of the communications between the ground and the aircraft even though they have little or no impact on the route followed.
During combinations of sectors (at night for example, several small sectors are combined into a single sector), it is necessary to rapidly warn each aircraft individually that it must change its frequency in order to match that of this new sector.
In the same manner, when the traffic in a sector becomes too heavy for a controller and a decision to divide the sector into two or more is made, it is necessary very rapidly to warn the aircraft that are approaching the newly created sector that they must change frequency.
Because the average number of aircraft per sector varies from 10 to 20 and communications are still verbal, there may be a considerable period of floating between two changes.
To this must be added the possibilities of forgetting, not understanding, line congestion, which all generate potential problems.
Because currently each frequency change gives rise to four verbal messages: one from the controller of sector N to assign the future frequency, followed by a response (check) from the pilot, followed by the pilot contacting the sector N+1 followed by the acquiescence of the controller N+1, problems of congestion, repetition, forgetting will necessarily play an increasing part, problems that are greatly amplified by the dynamic partition/combination of the sectors as is envisaged in Europe in the years following 2010 in order to improve the flexibility of the control sectors.